Wave Propagation and Dynamic Fracture in Laser Shock-Loaded Solid Materials

Abstract

Shock wave loading of solid materials results in specific damage processes at high strain rates. The most widely studied of these processes is probably spall fracture (e.g. Antoun et al., 2002, and references therein), which arises from tensile stresses generated by the interaction of release waves within the material upon reflection of a compressive pulse from a free surface or from an interface with a layer of lower acoustic impedance. If such tensile stresses exceed the dynamic strength of the material, they cause the nucleation and growth of micro-voids or micro-cracks which may eventually coalesce to form a macroscopic fracture and lead to the ejection of one or several fragments (spalled layers) from the sample. Spall damage and wave propagation are tightly coupled. On one hand, the creation of new free surfaces accompanying damage development induces stress relaxation which gives rise to recompression waves. Such waves can be detected in time-resolved velocity (Antoun et al., 2002; Tollier et al., 1998) or piezoelectric (De Resseguier et al., 1997) measurements, and their analysis provides very rich data on the fracture process (location, time and tensile stress at damage initiation, rate of the damage growth, thickness of the spalled layer...). On the other hand, spall fracture results from wave interaction, so that post-test observations of the residual damage in recovered samples (location, sizes and shapes of the damages zones, fracture surface morphology...) may provide key information on the propagation of compression and release waves prior to failure. In this chapter, we illustrate this second, more original statement with experimental results obtained under laser driven shock loading. Intense irradiation of an absorbing target by a high power pulsed laser produces the vaporization of a thin layer of material, transformed into a plasma cloud, whose expansion toward the laser source induces by reaction a compressive pulse into the solid target. The main specificity of this technique is the very short time of pressure application (typically a few ns) compared to other shock generators such as plate impacts or explosive loading, where the duration of the pressure load is usually of μs-order. This difference makes laser shocks less destructive than those more conventional techniques, and favours sample recovery for post-shock analyses of residual damage. In a first example, spall fracture observed in laser shock-loaded single crystal quartz provides very clear evidence of the strong effect of crystal anisotropy on stress wave Source: Wave Propagation in Materials for Modern Applications, Book edited by: Andrey Petrin, ISBN 978-953-7619-65-7, pp. 526, January 2010, INTECH, Croatia, downloaded from SCIYO.COM